1. Field of the Invention
The present invention relates to an LPP (laser produced plasma) type EUV (extreme ultra violet) light source apparatus which generates extreme ultra violet light to be used for exposing semiconductor wafers, etc.
2. Description of a Related Art
Recent years, as semiconductor processes become finer, photolithography has been making rapid progress to finer fabrication. In the next generation, microfabrication at 60 nm to 45 nm, further, microfabrication at 32 nm and beyond will be required. Accordingly, in order to fulfill the requirement for microfabrication at 32 nm and beyond, for example, exposure equipment is expected to be developed by combining an EUV light source generating EUV light having a wavelength of about 13 nm and reduced projection reflective optics.
As the EUV light source, there are three kinds of light sources, which include an LPP (laser produced plasma) light source using plasma generated by applying a laser beam to a target, a DPP (discharge produced plasma) light source using plasma generated by discharge, and an SR (synchrotron radiation) light source using orbital radiation. Among them, the LPP light source has advantages that extremely high intensity close to black body radiation can be obtained because plasma density can be considerably made larger, that the light emission of only the necessary waveband can be performed by selecting the target material, and that an extremely large collection solid angle of 2π to 5π steradian can be ensured because it is a point light source having substantially isotropic angle distribution and there is no structure surrounding the light source such as electrodes. Therefore, the LPP light source is considered to be predominant as a light source for EUV lithography requiring power of more than 100 watts.
FIG. 35 shows the outline of a conventional LPP type EUV light source apparatus. As shown in FIG. 35, the EUV light source apparatus includes a driver laser 101, an EUV light generating chamber 102, a target material supply unit 103, and laser beam focusing optics 104.
The driver laser 101 is a master oscillator power amplifier type laser apparatus which generates a drive laser beam to be used for exciting a target material.
The EUV light generating chamber 102 is a chamber in which EUV light is generated and which is evacuated by a vacuum pump 105 to facilitate turning the target material into plasma and prevent the EUV light from being absorbed. Furthermore, a window 106 for passing a laser beam 120 generated by the driver laser 101 into the EUV light generating chamber 102 is attached to the EUV light generating chamber 102. In addition, a target injection nozzle 103a, a target collecting cylinder 107, and an EUV light collector mirror 108 are located in the EUV light generating chamber 102.
The target material supply unit 103 supplies the target material to be used for generating EUV light into the EUV light generating chamber 102 through the target injection nozzle 103a which is a part of the target material supply unit 103. Target material, which has become unnecessary without laser beam irradiation, among the supplied target material is collected by the target collecting cylinder 107.
The laser beam focusing optics 104 includes a mirror 104a for reflecting the laser beam 120 emitted from the driver laser 101 toward the EUV light generating chamber 102, a mirror adjusting mechanism 104b for adjusting the position and angle (tilt angle) of the mirror 104a, a focusing element 104c for focusing the laser beam 120 reflected by the mirror 104a, and a focusing element adjusting mechanism 104d for moving the focusing element 104c along the optical axis of the laser beam. The laser beam 120 focused by the laser beam focusing optics 104 reaches the trajectory of target material through the window 106 and a hole formed in the midsection of the EUV light collector mirror 108. Thus, the laser beam focusing optics 104 focuses a laser beam 120 so as to make a focus on the trajectory of target material. As a result, the target material 109 is excited and turned into plasma, and EUV light 121 is generated from the plasma.
The EUV light collector mirror 108 is a concave mirror on the surface of which an Mo/Si film reflecting light having a wavelength of 13.5 nm, for example, at a high reflectance is formed, and reflects the generated EUV light 121 to focus it on an IF (intermediate focusing point). The EUV light 121 reflected by the EUV light collector mirror 108 passes through a gate valve 110 provided to the EUV light generating chamber 102 and a filter 111 which eliminates unnecessary light (electromagnetic waves (light) having a shorter wavelength than EUV light, and light having a longer wavelength than EUV light, e.g. ultra violet light, visible light, infrared light, etc.) from the light generated from the plasma to allow passage of only desired EUV light, e.g. light having a wavelength of 13.5 nm. EUV light 121 focused on the IF (intermediate focusing point) is then guided to an exposure unit or the like through transmission optics.
Since large energy is radiated from plasma generated in the EUV light generating chamber 102, temperature of the components in the EUV light generating chamber 102 rises due to this radiation. A technology of preventing such temperature rise of components is known (see Japanese Unexamined Patent Application Publication JP-P 2003-229298A, for example).
JP-P2003-229298A describes an X-ray generator including an X-ray source which turns a target material into plasma and radiates X-ray from the plasma, and a vacuum vessel which accommodates the X-ray source, and the X-ray generator is characterized by an inner wall made of a material having high absorption ratio for electromagnetic waves in a range from infrared to X-ray inside the vacuum vessel. According to the X-ray generator, the components in the vacuum vessel can be prevented from being heated unnecessarily due to radiation energy reflected and scattered by the inner wall of the vacuum vessel.
By the way, the plasma generated in the EUV light generating chamber 102 as shown in FIG. 35 diffuses with the passage of time, and a part of the diffused plasma scatters as atoms and ions. These atoms and ions are called debris and irradiated to the inner wall and structure of the EUV light generating chamber 102. Due to the irradiation of debris scattered from the plasma as described above and electromagnetic waves generated from the plasma, the following phenomena may occur.
(a) Atoms scattered from the plasma adhere to the surface of the window 106 on the internal side of the EUV light generating chamber 102. The atoms adhered to the surface of the window 106 on the internal side of the EUV light generating chamber 102 absorb a laser beam 120.
(b) Ions scattered from the plasma are applied to the surface of the window 106 on the internal side of the EUV light generating chamber 102, and the surface of the window 106 on the internal side of the EUV light generating chamber 102 is deteriorated (the surface becomes rough and not smooth). As a result, the window 106 becomes to absorb a laser beam 120 emitted from the driver laser 101.
(c) Ions scattered from the plasma are applied to the inner wall and structures of the EUV light generating chamber 102. Atoms scattered from the inner wall and structures of the EUV light generating chamber 102 by this spattering adhere to the surface of the window 106 on the internal side of the EUV light generating chamber 102. Atoms adhered to the surface of the window 106 on the internal side of the EUV light generating chamber 102 absorb a laser beam 120.
(d) The window 106 absorbs electromagnetic waves (light) having a short wavelength generated from plasma, and thereby, quality of its material is deteriorated. As a result, the window 106 becomes to absorb a laser beam 120.
(e) When the operation period of the EUV light source apparatus becomes long to some extent, the material of the window 106 is deteriorated or damaged by the irradiation of the laser beam 120 during this period. As a result, the window 106 becomes to absorb a laser beam 120.
When the phenomena of (a) through (e) have occurred, the energy for turning target material into plasma decreases and the efficiency of generation of EUV light 121 decreases.
Furthermore, when the window 106 and/or atoms adhered to the window 106 absorb laser beam 120, the temperature of the window 106 rises, and distortion occurs on the substrate of the window 106, and therefore, the focusing property decreases. Such reduction in the focusing property results in more reduction in the efficiency of generation of EUV light 121. In addition, when the distortion of the substrate of the window 106 grows, the distortion, in turn, results in damage of the window 106.
Furthermore, there is a case where a part (e.g. lens, mirror, etc.) of the laser beam focusing optics 104 is located inside the EUV light generating chamber 102. In such a case, the phenomena (a) through (e) may occur also on the part of the laser beam focusing optics 104 located in the EUV light generating chamber 102. In particular, in the case where a mirror which reflects the laser beam is located in the EUV light generating chamber 102, when the phenomena (a) through (e) occur on the mirror, the laser beam reflectance of the reflection-increasing coating on the reflecting surface of the mirror decreases. As a result, the energy for turning target material into plasma decreases and the efficiency of generation of EUV light 121 decreases.
When the phenomena (a) through (e) have occurred and the window 106 and/or the laser beam focusing optics 104 have been deteriorated, the deteriorated optical elements need to be replaced with new optical elements.
However, there has been a problem that since a laser beam 120 is focused on a plasma generating position (the trajectory of target material) in the EUV light generating chamber 102, it is not easy to know whether the window 106 and/or the laser beam focusing optics 104 have been deteriorated and therefore it is difficult to take prompt countermeasure (optical element replacement).
On the other hand, as factors responsible for destabilizing generation of the plasma and eventually fluctuating or decreasing efficiency of generation of EUV light 121, there is a problem of displacement of focusing point (focus) of the laser beam 120, in addition to deterioration of the window 106 and the laser beam focusing optics 104. The displacement of focusing point of the laser beam 120 is caused by alignment deviation of the laser beam focusing optics 104, pointing deviation of the driver laser 101, and so on. Alignment deviation of the laser beam focusing optics 104 is mainly caused by deformation of the optical elements included in the laser beam focusing optics 104 or deformation of the optical element holders holding such optical elements because heat load is applied to the optical elements and the optical element holders along with operation of the EUV light source apparatus. Furthermore, pointing deviation of the driver laser 101 is mainly caused by deformation of the elements or the components in the driver laser 101 because heat load is applied to the elements and the components along with operation of the EUV light source apparatus.
When displacement of focusing point of the laser beam 120 as described above occurs, a focusing spot size or an intensity distribution in the plasma generating position (on the trajectory of target material) becomes inadequate or the laser beam 120 deviates from target material, and the generation of plasma is destabilized, and eventually, the efficiency of generation of EUV light 121 is fluctuated or decreased.
Focusing point displacement of the laser beam 120 can be corrected by readjusting the alignment of the laser beam focusing optics 104 without replacement of optical elements. Thereby, the focusing point of the laser beam 120 can be restored to the original position (plasma generating position), plasma generation can be stabilized, and, in turn, the efficiency of generation of EUV light 121 can be restored to the original value.
However, there has been a problem that since the laser beam 120 is focused in the EUV light generating chamber 102 (plasma generation position) it is not easy to know whether focusing point displacement of the laser beam 120 occurred and therefore it is difficult to take prompt countermeasure (readjusting the alignment of the laser beam focusing optics 104).
Furthermore, in particular, as the light output of the EUV light source apparatus increases, the amount of generated debris increases, and the surfaces of various optical elements such as the concave mirror 108 of the EUV light collecting optics become contaminated easily as well as the window 106, and therefore, the deterioration state should be adequately known and the optical elements should be cleaned or replaced.